This project is designed to determine the relationships among DNA repair, chromosome structure and mutagenesis in Drosophila melanogaster. A mutation that increases the mutant frequency (a mutator) has been identified and characterized. The mutator greatly reduces the efficacy of repair of x-ray-induced chromosome breaks in oocytes, thereby allowing a previously undescribed repair pathway to be observed. By this newly identified repair pathway broken chromosome ends are "capped" with a new telomere. The new chromosome ends are protected from degradation, but are not replicated effectively; DNA sequences are lost from the capped ends. Quasi-stable derivatives of the capped ends have been isolated and found to have gained a telomere-specific DNA sequence, termed HeT, which has a structure resembling a retrotransposon. Preliminary evidence suggests that natural chromosome ends also recede because of incomplete DNA replication and occasionally lengthen by the acquisition of HeT. The rate of loss is balanced by the rate of transposition to maintain chromosome length. We are also developing a rapid assay for the mutator to facilitate genetic analysis and investigating cell specificity of mutator activity. The mutator gene has been mapped to chromosome region 62B12-C1 using a nested set of 150 deletions. A yeast artificial chromosome that spans the region has been subcloned to map the location of the deletion breakpoints and to identify a transcription unit that corresponds to the mutator gene.